Why does Social inequality happen?

When it comes to stratification, or anything doing with sociology in general, there are three theoretical perspectives and usually three different ways to describe a situation that must be taken into account.  The Functionalist theory, which states that all parts of life work together to create an organized society,
The Conflict theory, which says that society is based on those with power, and those who have none, and that it is always a constant struggle for wealth, things that you own, and  power, the ability to control others, and there is always conflict, and the Symbolic Interactionism, which is the interaction of people based on symbols and mutually understood gestures.

The Functionalist theory of Stratification says that the reason stratification exists is due to the need for highly qualified careers and the need for highly qualified people to fill those positions.  They are needed because they are more important than others.  A president's position is more important than a bus driver's, is it not?

Conflict theory of Stratification states that there is inequality because people are willing to take advantage of others.  The constant struggle of between those with power and wealth, and those with nothing. Karl Marx introduced two terms when it came to this, the Proletariat and the Bourgeoisie.  Proletariat were those who were powerless, and the Bourgeoisie were powerful.  Those with power and wealth  are able to spread their values and beliefs throughout a system and government.  Those without power may sometimes give in to the given beliefs, which is called False Consciousness.

Symbolic Interactionism shows that people will tend to be socialized to accept their social status and to believe that the existing system is all well.  People will also believe that that success is gained from hard work, and those who are not successful lack the motivation and ethics to deserve equality.